1. Field of the Invention
OPTICAL FIBRE COUPLER
This invention relates to optical fibre couplers.
2. Related Art
With the advent of high capacity optical networks the need for low loss coupling arrays is becoming increasingly important. Both 1.times.N and M.times.N devices are of interest, depending on the network architecture, which can be made arbitrarily large by concatenating elementary 1.times.2 or 2.times.2 couplers. Couplers such as these can be fabricated in a number of different ways, for example using planar technology or by using fused fibre techniques. Both of these approaches have their restrictions. Planar devices although very small in size can suffer from reflections and relatively high losses when pigtailed onto standard system fibres. Fused fibre devices can be fabricated directly from system fibre, thus overcoming the problem of reflections and coupling losses, but concatenating these devices to form large arrays requires considerable space which is at a premium in modern telecommunications networks.
The use of fused fibre technology has been made more attractive recently in modern telecommunications networks by the development of monolithic devices with many input-output ports, such as 3.times.3, 4.times.4 and 1.times.7 couplers. See for example: Mortimore, D. B.; "Monolithic 4.times.4 single-mode fused coupler", Electronics Letters 25, 10, pp 682-683, 1989 and Mortimore, D. B. and Arkwright, J. W.; "Monolithic wavelength flattened 1.times.7 single-mode fused coupler", Electronics Letters 25, 9, pp 606-607, 1989. These devices allow a considerable saving in space over arrays formed from elementary couplers having fewer ports.